1. Field of the Invention
This invention relates to a pulse shaping filter for .pi./4-shift QPSK(Quadrature Phase Shift Keying) modulator. In particular, this invention relates to a digital shaping filter for reducing memory required by a pulse shaping filter for the .pi./4-shift QPSK modulator of the digital mobile communication systems.
2. Description of the Prior Art
It is well-known that the .pi./4-shift QPSK modulation scheme is a scheme that has combined a conventional QPSK modulation scheme with an Offset QPSK (OQPSK) scheme, and it is mainly used in North America mobile communication system IS-54 and Japan digital cellular system JDC.
Since the conventional .pi./4-shit QPSK modulation scheme was realized with analog quadrature modulators, distortion and drift were incurred in signal phase, direct-current offset and gain, thus deteriorating the performance of the whole system.
In order to improve that problem, it has been proposed to realize the mobile communication system with digital integrated circuits.
Further, there has been an effort to reduce the cost, size and power consumption of mobile communication terminals in order to realize an economical mobile communication system.
In particular, such digital integrated circuits are suitable for miniaturizing the integrated circuit of a modulator and realizing more stable terminals.
It is assumed that an identical memory look up table scheme is used in all modulators described below.
When a conventional .pi./4-shift QPSK modulator shaping filter has employed a prescribed memory look up table scheme, the number of address bits is doubled, thereby requiring a large memory of 2.sup.2N *S words, where N is the number of shift register steps and S is the number of samples per symbol.
As above, a digital shaping filter used in the .pi./4-shift QPSK modulator of a prescribed memory look up table scheme requires 2 bits per symbol to represent a group of transmission signal points on the orthogonal coordinate system.
Therefore, when a shaping filter is realized by a prescribed memory look up table scheme, the memory required by the conventional .pi./4-shift QPSK modulator is .sub.2.sup.N times as large as that of a general QPSK modulator.
Considering the .pi./4-shift QPSK modulation scheme, every odd symbol requires 2 bits to represent a symbol, while every even symbol requires only one bit.
FIG. 1 shows the transmission signal points of .pi./4-shift QPSK modulation scheme.
As shown in FIG. 1, two bits are required to represent the amplitude values of the transmission signal points shown as `.sym.`, and the amplitude values of the transmission signal points shown as `` can be represented by one bit.
Such scheme is disclosed in a paper in IEICE TRANS. COMMUN. Vol. E77-B, July 1994, p. 921-p. 926, entitled "A New Fully-Digitalized .pi./4-Shift QPSK Modulator for Personal Communication Terminals" by SAKADA et al. In this scheme, when the number of shift register steps increases, a prescribed memory look up table method is used to reduce the volume of the required memory considerably. In this case, however, still a large memory is required.
Further, the scheme proposed by SAKADA et al. was so improved that it reduces the required memory 2.sup.3N/2 times that of the QPSK modulation scheme, but there is still a problem that the required memory is large for VLSI implementation.